Material handling implements of the type under consideration have been in existance for many years. These implements normally incorporate a vehicle that has a pair of lift arms pivoted at one end on a support portion of the vehicle. A material handling unit, such as a bucket, is pivotally supported on the opposite end of the respective lift arms. The lift arms, which define a boom, are raised and lowered with respect to the vehicle by a pair of fluid rams while the bucket or unit is pivoted on the end of the boom by a single or a pair of fluid rams.
In normal operation of this unit, the boom is placed in a lowered position and the bucket is positioned to have the lower edge or wall generally flat with respect to the ground and the vehicle is driven forwardly to fill the bucket. After the bucket is filled, it is tilted or rolled back with respect to the boom to maintain the contents within the bucket as it is raised to a position enabling dumping of the bucket to transfer the contents to some other location. As the boom is being raised, it is necessary for the bucket to be pivoted with respect to the boom to maintain the bucket in a generally level position and prevent spilling of the contents.
In small compact units of this type, one of the problems encountered is the fact that the bucket is in close proximity above the operator when the boom is in the fully raised position. This presents a considerable hazard if the operator should inadvertently engage the control lever to tilt the bucket rearwardly and possibly spill the contents onto the operator.
To alleviate this problem, many types of mechanisms have been proposed for preventing rearward tilting of the bucket when the boom is in the fully raised position. One type of mechanism incorporates a hydraulic system which prevents rearward pivoting of the bucket when the boom is in the fully raised position. Other types include elaborate mechanical linkages between the bucket and the boom and the boom and/or vehicle.
One of the problems encountered with most of the prior art devices that prevent tilting or "rollback" in the fully raised position is that the operator in many instances is not capable of manually overriding the system should the need arise. There are many instances in which the bucket should be tilted rearwardly in the fully raised position and other times when it is essential that such movement be precluded. Thus, many operators disconnect the entire rollback mechanisms so that the unit is capable of being operated as desired. This of course, is extremely hazardous and can result in serious injury to the operator and damage to the vehicle.